A few comments on Red’s video:
Here in Australia if it’s hot and I haven’t had rainfall such that the upper soil (10cm) moisture drops below 4% then I tend to begin deep watering or fertigating by adding nutrients to water to make the most of that water when I do water, especially if I’m hand watering.
Over summer it took 10-12 days on average for the soil moisture to drop below 4% after a rainfall event where I live, and after February it stayed there until the end of March. The garden was parched.
I use our governments Australian Landscape Water Balance site to help determine that 4% for my soil type, but I could otherwise use an accurate soil moisture meter or learn to read my soil by hand.
Below 4% is important because soil microbiology begins to go dormant and die, and because microbial mucilage glues soil aggregates together, those aggregates then begin to break down. This lack of soil moisture also exposes the soil to oxidation from organic acids, and when rewet makes soluble nutrients available to microbes and plants.
On the left is an example of an organic acid test I did on my prismatic clay subsoil with lemon juice, on the right, water. If you look closely you can see how many days it took for the water to evaporate by counting the brown iron oxide rings.
Organic soils contain many organic acids such as humic and fulvic acids that perform a similar function.
Organic acids in the topsoil and oxidation is why plants grow like crazy after a bit of a dry spell and then rain.
Microbes feed on these soluble nutrients and create CO2 respiration bursts at ground level that plant leaves can then transpire and help accelerate plant growth.
However these CO2 burst nutrient “priming effects” don’t happen if soil moisture is kept just above the 4% soil moisture (see (b) above). So short, regular watering will prevent these bursts, and maintaining a soil moisture below that needed for basal respiration levels will also mean soil microbial activity and respiration is below that required for adequate nutrient cycling. Meaning you either water and water regularly to maintain basal respiration, or you let the soil dry and deep water when it reaches that 4% to get a burst.
Here in Australia to restore nutrient cycling in pasture researchers are artificially performing a similar function by drying & photo-oxidizing strips of soils by bringing them to the soil surface with tools like the SoilKee renovator, this has a similar effect.
In India they take subsoil and dry it.
Clay particles that are oxidized also exchange aluminium (toxic to most plants) for ferrous iron in soils, which is needed for plants to produce phytochromes such as in blueberry plants and many fruit trees. When you change the soil pH to grow blueberries with epsom salts (magnesium sulphate) or iron sulfate, this ferrous iron (Fe2+) is what you’re making more available by lowering the pH. And these phytochrome pigments control growth and flowering in some plants.
On the topic of fertigating, last year scientists from Japan also found a drought pathway in plants for acetic acid (vinegar) that I’ve keep meaning to trial, but the paper is paywalled and I didn’t know the concentrations until now when I found this figure.
I’ve just looked up common $1.20 supermarket 2L white vinegar here in Australia. It’s about 4-4.2% glacial acetic acid and I calculate that diluting it 1 part to 140-145 parts water may give near 20 mM (milliMole). So a 2L bottle should be enough for 280L of water depending on your water quality if you wanted to try it.
It would be interesting to trial adding the vinegar before or after wilting point. Either way, if rain isn’t forecast, a watering with vinegar may help plants survive until there is another rain event, and it may be best applied when soil moisture hits 4% to also get a CO2 burst from the water.
[Audio gets better, kinda]